During the past year, we have investigated several aspects of T-macrophage interaction in the induction of helper cells, both in mice and using human blood cells in tissue culture. Molecular complexes of Ia antigen and immunogen (GRF) which are capable of inducing T helper cells were analyzed biochemically, using labelled antigen. These complexes were also used to analyze receptors on T cells, and it was found that free antigen, and absorbed ALS made T cell specific-anti 'MTLA' and binding of GRF to T cells. Using chimaeric mice we found that the host environment has a major influence in the differentiation of haemopoietic stem cells into T cells or antigen presenting cells. Non responder stem cells, if allowed to mature in a phenotypically responder environment yielded responder T cells but non-responder antigen presenting cells. These results imply that Ir genes are expressed both in the host where stem cells differentiate, and also in the antigen presenting accessory cells. Primed T cell restimulation was found to require either macrophages or GRF. This process was also genetically restricted. GRF was found to induce helper cells in vivo.